The invention relates to a method for operating an exhaust system of an internal combustion engine, wherein nitrogen oxide (NOx) is reduced by the use of a SCR catalytic converter, and wherein the ageing state of the SCR catalytic converter is monitored, as well as to a computer program and an open-loop and/or closed-loop control device for use in such a method.
Exhaust systems in motor vehicles are known from prior art, which are equipped with different devices for the after-treatment of exhaust gas in order to meet existing statutory requirements. The functional capability of such devices must be monitored with on-board means during an operation of the motor vehicle. Within the scope of a so-called “on-board diagnosis” (OBD), it can, for example, be necessary to monitor an SCR catalytic converter (Selective Catalytic Reduction) and if need be to recognize said catalytic converter as being defective.
The basic principle of the SCR catalytic converter is that nitrogen oxide molecules (NOx) are reduced on a catalyst surface in the presence of ammonia (NH3) as the reducing agent to elementary nitrogen. The reducing agent is allocated by a dosing device upstream of the SCR catalytic converter. The determination of the desired dosing rate takes place in an electronic open-loop and/or closed-loop control device, in which methods for the operation and the monitoring of the SCR catalytic converter are deposited.
The monitoring of an SCR catalytic converter can take place using at least one NOx sensor. NOx sensors currently available on the market exhibit a cross-sensitivity to ammonia (NH3). That means a sensor signal of the NOx sensor does not exclusively indicate the respective NOx concentration but rather a summed signal of the NOx and the NH3 concentration. In the case of a NOx sensor that is disposed downstream of the SCR catalytic converter, an increase of the sensor signal can, for example, indicate a decreasing NOx conversion rate (increase in the NOx concentration) as well as a breakthrough of pure ammonia (increase in the NH3 concentration). A direct distinction between NOx and NH3 is therefore not possible.
The incidence of pure ammonia downstream of the SCR catalytic converter (so-called NH3 slippage) should be prevented because ammonia in high concentration has a harmful effect on health.
Monitoring functions currently known on the market ascertain the efficiency of a NOx reduction (NOx conversion rate) with the aid of each NOx sensor upstream and downstream of the SCR catalytic converter. The SCR catalytic converter disposed upstream can thereby also be replaced by a model-based characteristic value. Due to the ageing of the SCR catalytic converter, the achievable conversion rate decreases with increasing operating time, and the NOx emissions downstream of said SCR catalytic converter increase accordingly. Based on predetermined limit values for admissible NOx emissions, a threshold value for the SCR efficiency can be determined. If said threshold value is undershot, a system error in the exhaust system is then suggested. The accuracy of this method is however limited by the insufficient accuracy of the available NOx sensors and cannot in many cases meet the standards of the statutory laws.
The German patent specification DE 10 2007 040 439 A1 describes a monitoring strategy for an SCR catalytic converter, in which a NH3 storage capacity of the SCR catalytic converter is ascertained. It was in fact discovered that the capability of the catalytic converter to adsorb NH3 can be used as a characteristic or indicator for the ageing of or the damage to said catalytic converter. When using this strategy, the SCR catalytic converter is initially filled with reducing agent up to the maximally achievable NH3 storage capacity by means of a superstoichiometric reducing agent dosing. In so doing, a defined starting point for a diagnosis is attained. The attainment of the maximum storage capacity is detected on the basis of a breakthrough of ammonia (NH3 slippage) through the catalytic converter. The NH3 slippage can be indirectly measured on account of the aforementioned cross-sensitivity of the NOx sensor to NH3.
The dosing of the reducing agent is subsequently reduced in relation to a normal dosing or is entirely turned off; thus enabling the stored NH3 mass to gradually be depleted again by means of the NOx reduction (so-called emptying test). By determining the SCR efficiency or other characteristic values dependent on the NOx conversion rate during the emptying test, the useable NOx storage capacity of the SCR catalytic converter can be indirectly determined, because with a lower stored NH3 mass, less NOx can be converted on the catalytic converter surface.
The disadvantage of this method is the long time period required for discharging the NH3 storage. In particular in the case of future exhaust gas after-treatment systems, in which a NOx storage catalytic converter is installed close to the engine for the purpose of further lowering the NOx emissions even when cold starting said engine, the NOx concentration can be so low upstream of the SCR catalytic converter that the time required for discharging the NH3 storage becomes too long, and this method can therefore no longer be used.